Generally, an intracavity-doubled laser comprises a laser diode for pumping a solid-state laser, such as Nd: YAG for example, forming an amplifier at 946 nm. In order to produce the doubling, a non-linear crystal is linked to the amplifier converting the near infrared fundamental signal into a visible signal by frequency doubling, (also known as “second harmonic generation SHG”). An harmonic wavelength equal to the fundamental wavelength divided by two is thus obtained. The amplifier and the non-linear crystal are contained in a cavity the two extreme opposite surfaces of which in the path of the laser beam are reflective for certain wavelengths.
However, if a continuous emission is sought, the power of the fundamental emission is less than the power of the laser diode and the frequency doubling is therefore very inefficient.
The U.S. Pat. No. 4,809,291, entitled “Diode pumped laser and doubling to obtain blue light” is known, in which R. L. Byer and T. Y. Fan propose an intracavity doubling in order to increase the power of a fundamental wavelength at 946 nm and thus increase the doubling efficiency.
In an article entitled “Efficient blue emission from an intracavity-doubled 946 nm Nd: YAG laser” published in 1988 in the journal Optics Letters (vol. 13, pp. 137-139); Dixon et al. present an emission of 5 mW of blue light (473 nm) by an intracavity-doubled Nd: YAG-based microlaser. The Nd concentration is 1.1 at. %. The doubling efficiency is only 2%.
The main problem with these intracavity-doubled lasers is the presence of axial modes and of spurious polarization which reduce the efficiency of the laser and which are the source of high power fluctuations. As an example, Matthews et al., in an article entitled “Diode pumping in a blue (473 nm) Nd:YAG/KNb03 microchip laser” (CLEO'96, vol. 9, p. 174) produce 26.5 mW of blue light with fluctuations of intensity greater than 10%.
More precisely, the intracavity frequency doubling causes selective losses which increase with the pumping power for the main laser emission. When the doubling efficiency increases, the average population inversion of the cavity must increase in order to compensate for the excess loss. However, this allows adjacent modes and the orthogonal polarization emission to start to lase. For the adjacent modes, this effect is in addition to that of “spatial hole burning” which already allows the adjacent modes to lase.
The different modes lasing in the cavity are coupled in the amplifying medium (gain competition) and in the frequency doubling medium (frequency addition). These couplings are non-linear and participate in a complex non-linear dynamic. The latter results in a high or even chaotic fluctuation of power.
If the frequency doubling is of “Type I”, the orthogonal polarization modes are not subject to efficient frequency doubling (absence of phase adaptation between the fundamental and the harmonic). These modes stabilize the population inversion by increasing with the pumping power. They slow the conversion efficiency which requires an increase of the population inversion in order to increase. Only “spatial hole burning” effects allow a slight increase in the conversion efficiency.
Several methods have been presented for making the laser monomode or for uncoupling the modes in the non-linear crystal. They can be separated into three categories:
a) The first is the introduction of an etalon into the cavity. This method, disclosed in particular in the U.S. Pat. No. 5,838,713 of Y. Shimoji, poses several problems. The etalon causes losses in the cavity unless it is formed by the faces of the YAG and of the doubling crystal. In the latter case, it requires very great precision in positioning (sub-micrometric) which is difficult to obtain industrially and to stabilize. A way of solving this problem is to bring the amplifying medium into optical contact with the doubling crystal incorporating an angle on one portion of the contact face. This angle produces a small air gap between the two materials. This method weakens the contact and therefore the integrity of a monolithic laser and does not allow the protection of the interface by a bonding agent.
b) The second category involves the polarization of the fundamental. The amplifying medium can be inserted between two quarter wave plates in order to avoid the “spatial hole burning” effect, see in particular G. Hollemann et al., in “Frequency-stabilized diode-pumped Nd: YAG laser at 946 nm with harmonics at 473 nm and 237 nm”, Opt. Lett. 19, p. 192, February 1994. One drawback of this method is the introduction of losses into the cavity.
By Type I doubling, is meant an embodiment in which the fundamental laser beam propagates along one of the optical axes of the crystal (in general the slow axis) and the harmonic laser beam propagates along the other optical axis of the crystal, orthogonal to the first. Type I doubling occurs when it is possible to cut the crystal so that the refractive index of an optical axis at the fundamental wavelength is equal to the refractive index of the other optical axis at the harmonic wavelength. This is the case for KNbO3.
By type II doubling, is meant an embodiment in which the fundamental laser beam is present on the two axes and the conversion coefficient is optimized when the polarization of the fundamental laser forms an angle of 45° with respect to the optical axes.
c) The third method consists in reducing the length of the cavity. It was proposed by A. Mooradian in the U.S. Pat. No. 5,256,164 October 1993.
For a linewidth of 1 nm for emission at 946 nm (compared with 0.6 nm for the line at 1.064 μm), Mooradian's formula requires a cavity length of less than 300 μm, including the YAG and the KNbO3. The Nd concentration in the microchips published or patented to date does not exceed 1.1 at. %. This corresponds to an attenuation of 0.85 mm−1 at 808.4 nm, i.e. 8.1% of absorbed pump power per 100 μm of thickness and 15.60% of absorbed pump power per 200 μm. However, the 100 or 200 nm of KNbO3 do not provide adequate conversion efficiency. Thus, a microchip laser according to Mooradian's inequality does not appear to be able to emit more than a few mW of blue light with laser diode pump power of 1 W.
The document “Low-noise diode-pumped intracavity-doubled laser with off-axially cut Nd:YVO4”, Opt. Lett. 19, p. 1624 (K. Suzuki et al.) describing the use of a walk-off in combination with a planoconcave lens. A device of this kind does not provide high reliability.
Moreover, an efficient method proposed by T. Y. FAN., “Single-Axial Mode, Intracavity Doubled Nd: YAG Laser”, IEEE Journal of Quantum Electronics, vol. 27, 9 Sep. 1991, is known for making an intracavity-doubled single-frequency laser. In this method, the amplifying medium (Nd: YAG) is cut at the Brewster angle with respect to the air. The non-linear, birefringent crystal is struck at 45° by the fundamental (type II doubling). The two Brewster angles causes significant losses in the orthogonal polarization and prevents it from lasing. It also causes losses at every wavelength at which the polarization has been rotated by the birefringent crystal. This loss modulation as a function of wavelength can make the laser monomode. On the other hand, this method does not apply to a Type I frequency doubling as, on principle, the signal at the fundamental frequency is on one of the optical axes of the non-linear crystal. However, because of the double refraction, it is not possible to join the amplifying crystal cut at the Brewster angle to the non-linear crystal. In fact, the double refraction introduces phase effects which mean that the beams reflected by the external face of the cavity do not recombine when they return to the amplifier. Finally, the main difficulty of the design proposed by FAN is maintaining the total length of the cavity with a precision of better than a few hundreds of nanometers in order to avoid any mode jump and its subsequent power instabilities. In fact, an increase (or reduction) in the cavity length of λ/4 (i.e. approximately 250 nm) makes it possible to change from a monomode to a bimode function. A additional increase in length of λ/4 makes it possible to return to a monomode function on the adjacent mode.